Earth Fault Detection Apparatus

BACKGROUND

Technical Field

The present invention relates to an earth fault detection apparatus using a flying capacitor.

Related Art

In a vehicle such as a hybrid vehicle including an engine and an electric motor as driving sources or an electric vehicle, a battery equipped in a vehicle body is charged to generate a propulsive force with use of electric energy supplied from the battery. A battery-related power supply circuit is typically configured as a high-voltage circuit that handles a high voltage equal to or higher than 200 V. In order to ensure its safety, the high-voltage circuit including a battery has a non-grounded configuration electrically insulated from the vehicle body, which serves as a reference potential point of grounding.

In a vehicle equipped with a non-grounded high-voltage battery, an earth fault detection apparatus is provided to monitor an insulation state (earth fault) between the vehicle body and a system provided with the high-voltage battery, specifically, a main power supply system from the high-voltage battery to a motor. As the earth fault detection apparatus, a method of using a capacitor called a flying capacitor is widely used.

In order to obtain knowledge about an insulation resistance, the earth fault detection apparatus of a flying capacitor type selectively switches measurement paths via switches, each of the measurement paths including a detection capacitor functioning as the flying capacitor, wherein the earth fault detection apparatus performs a V 0 measurement, a Vc 1 n measurement, and a Vc 1 p measurement. Here, the V 0 measurement is a measurement of a voltage corresponding to the voltage of the high-voltage battery. The Vc 1 n measurement is a measurement of a voltage reflecting influence of RLn, which is an insulation resistance on the negative side. The Vc 1 p measurement is a measurement of a voltage reflecting influence of RLp, which is an insulation resistance on the positive side.

It is known that the insulation resistance obtained by combining the insulation resistance on the positive side and the insulation resistance on the negative side together can be calculated from V 0 , Vc 1 n , and Vc 1 p that have been obtained through these measurements. The operation for the calculation is complicated. Hence, a conversion map for converting the insulation resistance with Vc 1 /V 0 (where Vc 1 =Vc 1 p +Vc 1 n ) as a reference value is generally prepared in advance. The insulation resistance is then estimated from the value of Vc 1 /V 0 that has been obtained through the measurements with reference to the conversion map, and an occurrence of an earth fault is determined.

In any of the measurements of V 0 , Vc 1 n , and Vc 1 p , a charging voltage, at the time when the detection capacitor has been charged for a predetermined charging time Δtm, is measured and set as a measurement value. The charging time Δtm is set to such a value that the detection capacitor is not fully charged in order to determine the earth fault in a short time. On the other hand, in a case where the charging time Δtm is too short, the charging voltage decreases, and the detection accuracy decreases because of influences of noise and the like. For this reason, the charging time Δtm is determined in consideration of the balance between the determination time and the detection accuracy.

Patent Literature 1: JP 2017-78587 A

SUMMARY

It is possible to calculate the conversion map for obtaining the insulation resistance from the reference value that has been obtained from the respective measurement results of the V 0 measurement, the Vc 1 n measurement, and the Vc 1 p measurement, based on the capacitance of the detection capacitor, the value of the charging resistor provided on the measurement path, and the charging time Δtm.

The values of the charging resistor and the charging time Δtm can be set to stable values with few variation. However, the capacitances of the individual detection capacitors differ from each other relatively largely, and in addition, changes under an influence of a surrounding environment such as temperature.

In a case where the capacitance of the detection capacitor when measured changes from the capacitance of the detection capacitor set when the conversion map is created, the accuracy of the insulation resistance to be calculated may be degraded.

Therefore, the present invention has an object to prevent a decrease in calculation accuracy of an insulation resistance, even when the capacitance of a detection capacitor changes, in an earth fault detection apparatus of a flying capacitor type.

In order to address the above issue, an earth fault detection apparatus in the present invention is an earth fault detection apparatus to be connected with a non-grounded battery and for calculating an insulation resistance of a system including the battery, the earth fault detection apparatus including:

a capacitor configured to operate as a flying capacitor;

a switch group configured to switch between a first measurement path, a second measurement path, and a third measurement path, the first measurement path including the battery and the capacitor, the second measurement path including the battery, a negative-side insulation resistance that is an insulation resistance between a negative-side line of the battery and ground, and the capacitor, the third measurement path including the battery, a positive-side insulation resistance that is an insulation resistance between a positive-side line of the battery and the ground, and the capacitor; and

a control unit configured to calculate a reference value based on each charging voltage of the capacitor in each of the first to third measurement paths, and configured to calculate the insulation resistance with reference to a predetermined conversion map, in which

the conversion map includes a conversion map corresponding to a capacitance of the capacitor, and

the control unit estimates the capacitance of the capacitor, and refers to the conversion map corresponding to the capacitance of the capacitor that has been estimated.

According to the present invention, in the earth fault detection apparatus of the flying capacitor type, even when the capacitance of the detection capacitor changes, a decrease in the calculation accuracy of the insulation resistance can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of an earth fault detection apparatus according to an embodiment of the present invention;

FIGS. 2 A and 2 B are diagrams for describing a variation and a detection error of a detection capacitor;

FIG. 3 is a flowchart for describing an earth fault determination operation of the earth fault detection apparatus;

FIG. 4 is a flowchart for describing a method for estimating a capacitance of the detection capacitor based on an ambient temperature; and

FIG. 5 is a diagram for describing a method for estimating a capacitance of the detection capacitor through actual measurements.

DETAILED DESCRIPTION

Embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram illustrating a configuration of an earth fault detection apparatus 100 according to an embodiment of the present invention. As illustrated in the drawing, the earth fault detection apparatus 100 is an apparatus of a flying capacitor type that is connected with a high-voltage battery 300 and that detects an earth fault of a system provided with the high-voltage battery 300 . Here, RLp represents an insulation resistance between the positive side of the high-voltage battery 300 and the ground, and RLn represents an insulation resistance between the negative side thereof and the ground.

The high-voltage battery 300 is a battery used for driving a vehicle to travel. The high-voltage battery 300 is configured with a rechargeable battery such as a lithium ion battery, and drives a load 310 including e.g. an electric motor.

As illustrated in the drawing, the earth fault detection apparatus 100 includes a detection capacitor C 1 that operates as a flying capacitor, and a control device 120 .

In addition, the earth fault detection apparatus 100 includes four switches S 1 to S 4 in the surroundings of the detection capacitor C 1 in order to switch the measurement paths and to control charging and discharging of the detection capacitor C 1 . These switches each can be configured with an insulating switching element, such as an optical MOSFET.

The switch S 1 has a first end connected with a positive-side power supply line and a second end connected with an anode side of a diode D 1 . A cathode side of the diode D 1 is connected with a first end of a resistor R 1 , and a second end of the resistor R 1 is connected with a first pole of the detection capacitor C 1 . The switch S 2 has a first end connected with a negative-side power supply line and a second end connected with a first end of a resistor R 2 . A second end of the resistor R 2 is connected with a second pole of the detection capacitor C 1 .

The switch S 3 has a first end connected with a first end of a resistor R 3 and an anode side of a diode D 3 , and a second end connected with a first end of a resistor R 5 , an analog input terminal of the control device 120 , and a first pole of a capacitor C 2 having a second pole that is grounded. A cathode side of the diode D 3 is connected with the first pole of the detection capacitor C 1 , a second end of the resistor R 3 is connected with a cathode side of a diode D 2 , and an anode side of the diode D 2 is connected with the first pole of the detection capacitor C 1 . A second end of a resistor R 5 is grounded. The switch S 4 has a first end connected with the second pole of the detection capacitor C 1 and a second end connected with a first end of a resistor R 4 . A second end of the resistor R 4 is grounded.

The control device 120 is configured with a microcomputer or the like, and executes a program that has been incorporated beforehand to conduct various types of control required for the earth fault detection apparatus 100 . Specifically, the control device 120 controls the switches S 1 to S 4 to switch the measurement paths individually, and also controls charging and discharging of the detection capacitor C 1 .

The respective measurement circuits for the V 0 measurement, the Vc 1 n measurement, and the Vc 1 p measurement are similar to those in the related art. That is, in the V 0 measurement, the switches S 1 and S 2 are turned on to charge the detection capacitor C 1 . In the Vc 1 n measurement, the switches S 1 and S 4 are turned on to charge the detection capacitor C 1 . In the Vc 1 p measurement, the switches S 2 and S 3 are turned on to charge the detection capacitor C 1 .

In addition, the control device 120 receives an input of an analog level corresponding to the charging voltage of the detection capacitor C 1 from an analog input terminal, wherein the analog level serves as a measurement value. A decrease in the insulation resistance of the system provided with the high-voltage battery 300 is detected based on such a measurement value. When the charging voltage for the detection capacitor C 1 is applied to the analog input terminal, the switches S 3 and S 4 are turned on. In this path, the detection capacitor C 1 is also discharged.

The above description relates to a basic circuit configuration widely used in the earth fault detection apparatus of a flying capacitor type. However, the earth fault detection apparatus of the flying capacitor type has various modifications. The configuration of the earth fault detection apparatus 100 is not limited to the example in the drawing.

In the present embodiment, the control device 120 further includes a detection capacitor capacitance estimation unit 130 and a conversion map 140 . For example, the detection capacitor capacitance estimation unit 130 can be configured by the control device 120 executing a predetermined program, and the conversion map 140 can be configured in a storage area included in the control device 120 .

The detection capacitor capacitance estimation unit 130 is a block that estimates a current capacitance of the detection capacitor C 1 . A method for estimating the current capacitance of the detection capacitor C 1 is not limited, and various methods are adoptable.

For example, the capacitance of the capacitor is generally influenced by temperature. Hence, it is possible to measure the temperature around the detection capacitor C 1 using a thermometer TM131 (see FIG. 1 ), and to calculate a change from the initial capacitance for estimation. In addition, the capacitance of the detection capacitor C 1 may be estimated through actual measurements. For example, the detection capacitor C 1 that has been charged is discharged for a predetermined time, and the capacitance can be calculated based on a change in the charging voltage before and after the discharge. In this case, the thermometer TM131 may be omitted. A procedure for estimating the current capacitance of the detection capacitor C 1 will be described later.

The conversion map 140 is a map for converting a reference value obtained from the V 0 measurement, the Vc 1 n measurement, and the Vc 1 p measurement into a value of the insulation resistance. The conversion map 140 is created to correspond to the capacitance of the detection capacitor. For example, the capacitance can be classified into a plurality of groups in accordance with its magnitude, and the conversion map 140 can be created for each group. For each group, a capacitance representing the group is determined. For example, it is possible to set the capacitance representing the median value of the group defined by an upper limit capacitance and a lower limit capacitance.

The conversion map 140 can be calculated, based on the capacitance of the detection capacitor C 1 , the values of the charging resistors (R 1 , R 2 ) provided in the measurement path, and the charging time Δtm. Here, the values of the charging resistors and the charging time Δtm are fixed, and the capacitance of the detection capacitor C 1 is changed by the capacitance representing the group to create the conversion map 140 for each group. The conversion map 140 includes a standard conversion map 140 that has been created to correspond to the initial capacitance of the detection capacitor C 1 .

After calculating the reference value based on the V 0 measurement, the Vc 1 n measurement, and the Vc 1 p measurement, the control device 120 calculates the insulation resistance with reference to the standard conversion map 140 or the conversion map 140 corresponding to the estimated value of the current capacitance of the detection capacitor C 1 .

The earth fault detection apparatus 100 in the present embodiment calculates the insulation resistance with reference to the conversion map 140 corresponding to the estimated value of the current capacitance of the detection capacitor C 1 . Therefore, even when the capacitance of the detection capacitor changes, a decrease in the calculation accuracy of the insulation resistance can be prevented.

Here, a variation and a detection error of the detection capacitor C 1 will be described with reference to FIGS. 2 A and 2 B . FIG. 2 A illustrates changes in the reference value with respect to the insulation resistance of a case where the capacitance of the detection capacitor increases from an initial capacitance B to a capacitance A, and a case where the capacitance decreases to a capacitance C. As illustrated in the diagram, differences occur in the reference value for each capacitance between a region where the insulation resistance is low and a region where the insulation resistance is high, and such differences are remarkably large in the region where the insulation resistance is low.

FIG. 2 B illustrates a detection error in a case where the insulation resistance is calculated with a standard conversion map corresponding to the initial capacitance B, based on the reference value that has been obtained in each case of the capacitance A, the capacitance B, and the capacitance C. In the region where the insulation resistance is low, the differences in the reference value depending on the capacitance are obvious, and the ratio of the error to the true insulation resistance increases, and the detection error is extremely large. Theoretically, the error becomes zero at a resistance value Rt at which the insulation resistance is equal to the charging resistor. However, when the insulation resistance becomes lower than it, the error becomes abruptly worse, and the calculation accuracy decreases.

Conversely, unless the insulation resistance decreases as low as the charging resistor, the change in the capacitance of the detection capacitor C 1 does not largely affect the calculation accuracy of the insulation resistance.

For this reason, in the present embodiment, in a case where there is a possibility that the insulation resistance is decreased, the current capacitance of the detection capacitor C 1 is estimated, and the conversion map 140 corresponding to the current capacitance of the detection capacitor C 1 that has been estimated is referred to. On the other hand, in a case where there is no possibility that the insulation resistance is decreased, the current capacitance of the detection capacitor C 1 is not estimated, and the standard conversion map 140 is referred to.

Accordingly, it is sufficient to prepare the conversion map 140 corresponding only to the region where the insulation resistance is decreased except for the standard conversion map 140 . Therefore, the storage area for the conversion map 140 in the control device 120 can be saved.

However, regardless of the possibility that the insulation resistance is decreasing, the current capacitance of the detection capacitor C 1 may be estimated, and the conversion map 140 corresponding to the current capacitance of the detection capacitor C 1 that has been estimated may be referred to. Accordingly, the calculation accuracy of the insulation resistance can be further improved.

Next, an earth fault determination operation by the earth fault detection apparatus 100 having the above configuration, will be described with reference to a flowchart of FIG. 3 . This operation is performed under the control of the control device 120 .

First, opening and closing of the switches S 1 to S 4 are controlled, and the V 0 , Vc 1 n , and Vc 1 p measurements are respectively performed (S 101 ). Then, a reference value is calculated, based on V 0 , Vc 1 n , and Vc 1 p that have been obtained through the measurements (S 102 ).

Whether there is a possibility that the insulation resistance is decreasing is determined, based on V 0 , Vc 1 n , and Vc 1 p that have been obtained through the measurements (S 103 ). As described above, an error increases when the insulation resistance is lower than the charging resistor. Therefore, for example, a case where V 0 <(Vc 1 n +Vc 1 p ) is established can be considered as an indication that the insulation resistance is decreased.

As a result, in a case where it is determined that there is the possibility that the insulation resistance is decreasing (S 103 : Yes), the detection capacitor capacitance estimation unit 130 estimates the current capacitance of the detection capacitor C 1 (S 104 ). The procedure for estimating the current capacitance of the detection capacitor C 1 will be described later.

Then, with reference to the conversion map 140 corresponding to the capacitance of the detection capacitor C 1 that has been estimated (S 105 ), the insulation resistance is calculated (S 107 ). After the insulation resistance is calculated, an earth fault is determined, based on whether the insulation resistance that has been obtained is lower than a predetermined reference value (S 108 ).

On the other hand, in a case where it is determined that there is no possibility that the insulation resistance is decreasing (S 103 : No), with reference to the standard conversion map 140 (S 106 ), the insulation resistance is calculated (S 107 ). After the insulation resistance is calculated, an earth fault is determined, based on whether the insulation resistance that has been obtained is lower than a predetermined reference value (S 108 ).

Finally, a description will be given with regard to an example of estimating, by the detection capacitor capacitance estimation unit 130 , the current capacitance of the detection capacitor C 1 . First, a procedure for an estimation method based on an ambient temperature of the detection capacitor C 1 will be described with reference to a flowchart of FIG. 4 .

In the present method, the actual capacitance of the detection capacitor C 1 at a reference temperature is measured before shipment of the earth fault detection apparatus 100 . The actual capacitance of the detection capacitor C 1 can be calculated by, for example, discharging the charged detection capacitor C 1 for a predetermined time and measuring a change in charging voltage before and after the discharge.

Then, the estimated capacitance of the detection capacitor C 1 for each ambient temperature is actually measured or theoretically obtained together with the actual capacitance, and is written as a capacitance table in a storage area in the control device 120 that can be referred to by the detection capacitor capacitance estimation unit 130 . Instead of the capacitance table, a calculation formula or the like for calculating the present capacitance of the detection capacitor C 1 from the ambient temperature may be written.

When the present capacitance of the detection capacitor C 1 is estimated, the ambient temperature of the detection capacitor C 1 is acquired with use of the thermometer TM131 (S 201 ). Then, the capacitance table that has been written before the shipment is referred to (S 202 ), and the capacitance corresponding to the ambient temperature is estimated as the present capacitance of the detection capacitor C 1 (S 203 ).

Next, a method for estimating the present capacitance of the detection capacitor C 1 through actual measurements will be described with reference to FIG. 5 . Here, the detection capacitor C 1 that has been charged is discharged for a predetermined time, and the present capacitance of the detection capacitor C 1 is calculated, based on a change in the charging voltage before and after discharge. This technique is described in JP 2017-78587 A, only its outline will be described.

As illustrated in the drawing, in a case where the detection capacitor C 1 is charged between a time t 0 and a time t 1 and discharged between a time t 1 and a time t 2 , the ratio of a charging voltage V 2 at the time t 2 to a charging voltage V 1 at the time t 1 depends on the capacitance of the detection capacitor C 1 . Note that it is assumed that the charging resistor is known and constant.

For this reason, the detection capacitor capacitance estimation unit 130 controls openings and closings of the switches S 1 to S 4 , charges and discharges the detection capacitor C 1 for a predetermined time, and measures the charging voltage V 1 of the detection capacitor at the time of ending the charging and the charging voltage V 2 of the detection capacitor at the time of ending the discharging. Then, the current capacitance of the detection capacitor C 1 is calculated, based on the charging voltages that have been measured and the resistance related to the discharging time.

Specifically, C 1 =(t 2 −t 1 )/(Rd×ln (V 2 /V 1 )) can be used for calculation, where Rd represents the resistance related to the discharging time.

As described heretofore, the earth fault detection apparatus 100 in the present embodiment estimates the current capacitance of the detection capacitor C 1 , and calculates the insulation resistance with reference to the conversion map 140 corresponding to the estimated value. Therefore, even when the capacitance of the detection capacitor changes, a decrease in the calculation accuracy of the insulation resistance can be prevented.

In this situation, in a case where there is no possibility that the insulation resistance is decreasing, the standard conversion map 140 may be referred to without an estimation of the current capacitance of the detection capacitor C 1 . Accordingly, the storage area for the conversion map 140 in the control device 120 can be saved.

REFERENCE SIGNS LIST

• 100 Earth fault detection apparatus • 120 Control device • 130 Detection capacitor capacitance estimation unit • 140 Conversion map • 300 High-voltage battery • 310 Load